Magnetic materials are used in many different compact devices and systems. The magnetic materials of interest for these systems exhibit a ferromagnetic or ferrimagnetic response, and more specifically, a large magnetization in response to an applied magnetic field. These magnetic materials are generally sub-categorized into either soft magnetic materials, which have a lower coercivity (generally <1 kA/m), or hard magnetic materials, which have a higher coercivity (generally >10 kA/m).
Soft magnetic materials are used in a wide array of electronic devices. For example, power electronic circuits for power conversion and regulation often include magnetic passives such as inductors and transformers, which use soft magnetic materials in the inductor and transformer cores. Furthermore RF/microwave radio circuits for wireless connectivity utilize inductors and transformers, as well as phase shifters, circulators, and isolators, which also employ soft magnetic materials. These magnetic passives tend to be the largest, heaviest, and/or most inefficient system components of the circuits to which they form a part.
Limitations of magnetic materials used in soft magnetic cores for forming magnetic passives include magnetic saturation and core loss, particularly at high operational frequencies (1 MHz-10 GHz). In addition, core loss may be dominated by eddy current losses.
Hard magnetic materials (permanent magnet behavior) are also used in a wide variety of devices. For example, hard magnets supply the magnetic fields for electrodynamic or magnetic transduction in actuators, energy-converters, motors, and generators. Hard magnets are also used to provide bias fields or a fixed magnetic moment for magnetic field sensors, proximity sensors, biomedical devices, and other devices where a stable magnetic field is required. Hard magnets are also used to form magnetic latches.
Limitations of magnetic materials used in hard micromagnets include coercivity, remanence, maximum energy product, chemical stability (e.g. propensity for oxidation or corrosion), and temperature sensitivity.
One challenge in forming small-scale magnetic devices and systems is related to the dimensional requirements of the necessary magnetic materials. Magnetic structures with critical dimensions ranging from ones of micrometers up to hundreds of micrometers are desired, but these structural dimensions fall in a “technology gap” between thin-film processing and bulk manufacturing.
That is, bottom-up thin-film deposition processes do not easily yield thick enough magnetic layers (e.g., on the order of micrometers). Conversely, top-down machining of fine-scale structures from bulk materials is challenging, and assembly of these structures into functional devices requires extensive packaging overhead, which increase size and cost. Thus, it is difficult to achieve thick magnetic materials with good magnetic properties in a highly manufacturable process, since it is challenging to achieve both manufacturability and performance at the same time.
Because of this technology gap, magnetic materials remain notoriously difficult to integrate into planar substrate manufacturing processes such as wafer-level electronics manufacturing or printed-circuit-board manufacturing. Hence, these wafer-level and board-level integration challenges inhibit the use of magnetic materials in the formation of small-scale magnetic devices and systems.
Another challenge in manufacturing small-scale magnetic devices and systems in a manner that enables high-throughput as well as compatibility and integration with existing manufacturing platforms is that the material properties of a magnetic material are strongly coupled with the material synthesis/deposition process. Changing the thickness of a deposited magnetic film can alter the resulting material properties due to, for example, stoichiometry differences, stress, and shape anisotropy.